1. Field of the Invention
The embodiments described below are directed to methods for fabricating MOSFET semiconductor devices, and more particularly to methods for prolonging MOSFET device lifetimes.
2. Background of the Invention
As is well understood, a MOSFET device is formed over a silicon substrate layer. The silicon substrate layer can be a P-type silicon layer or an N-type silicon layer; however, for illustration, P-type silicon substrates are addressed here. A drain and source region are then implanted in the substrate. The drain and source region are silicon regions that are doped to be of the opposite type as the substrate. For example, where a P-type silicon substrate is used, N-type drain and source regions can be implanted therein. A charge trapping stack can then be formed over the substrate between the drain and source regions, and a gate electrode is formed over the charged trapping stack.
The charged trapping stack typically comprises a dielectric layer formed over the substrate, a charged storage layer formed on the dielectric, and another dielectric layer formed over the charge storage layer. Charge can be stored in the charge storage layer by applying the appropriate voltages to the gate electrode, drain, source, and substrate. With the proper voltages, electrons will be induced to flow in the substrate between the drain and source regions. A certain number of the electrons will penetrate through the bottom dielectric layer and into the charged storage layer where they will be trapped. The bottom dielectric layer is sometimes referred to as a tunnel dielectric layer.
Unfortunately, the interface between the substrate and the bottom dielectric in a conventional MOSFET device often comprises silicon dangling bonds that are passivated by hydrogen after conventional processes. The silicon-hydrogen bonds are weak and can easily be broken by hot carrier stressing, i.e., the high energy carriers can cause Si—H bond breaking at the silicon-dielectric interface.
The breaking of the silicon-hydrogen bonds can reduce performance over time, and therefore reduce the lifetime of the device. Increased lifetime for MOSFET devices is clearly important and as a result, the reduced lifetime produced by the silicon-hydrogen bonds referred to above is clearly a disadvantage.